Purpose: To construct and experimentally validate a compensator-based brachytherapy (CBT) prototype system for treating cervical cancer. With CBT, high-precision 3-D manufacturing techniques are used to generate a patient-specific compensator with spatially-varying thicknesses, through which an electronic brachytherapy (eBT) source travels to deliver conformal radiation dose distributions.

Methods: A prototype cylindrical compensator with 8 octants, each with different thicknesses, was designed. Direct Metal Laser Sintering (DMLS) was used to construct Cobalt-Chromium (CoCr) and Titanium (Ti) compensators, and a 4-axis CNC milling (4-D milling) technique was used to construct a Ti compensator. Gafchromic EBT2 films, held by an acrylic quality assurance (QA) phantom, were irradiated to approximately 125 cGy with an eBT source for both shielded and unshielded cases. The dose at each point on the films was calculated using a TG-43 dose calculation model that was modified to account for the presence of a compensator by ray-tracing.

Results: Both 4-D milling and DMLS methods achieved 25.4 μm and 20-40 μm spatial accuracy, respectively. Construction time is approximately 10 hours for 4-D milling and 2 hours for DMLS. The agreement between the expected EBT2 film dose and measured dose was within 10% without a compensator, and within 15% for non-thickness-transition area with a compensator. The use of Monte Carlo dose calculations is expected to improve dosimetric accuracy relative to the current TG-43-based method.

Conclusion: We expect that patient-specific compensators can be constructed rapidly in clinical situations using DMLS. CBT is a non-invasive alternative to supplementary interstitial BT that is expected to improve the dose conformity to bulky cervical cancer tumors relative to conventional intracavitary brachytherapy.

Funding Support, Disclosures, and Conflict of Interest: University of Iowa Research Foundation